It can take people different ages to conceive. I had a frined that tried to get pregnant when she was 19-20-21 with her husband and it never happened. Later on they got a divorce and she got remarried and had one child when she was about 30. It takes some people a lot longer than others to conceive ans everything happens for a reason, maybe it is not your time to have a baby. Everything happens for a reason!
A lot of people have problems conceiving but as some reasurrance one of my friends Trenda had a baby when she was 19. She started trying to have a baby when she was 17 and always worried about it but you have to understand that it can be harder to have a baby when you are younger. My friend Trenda was small built and so it was hard for her, I also have a friend that is overweight and it is hard for her also but it finally happened after 3 years but she had a misscarriage and then finally had a healthy pregnancy so hang in there okay.
You said that you are ready to have a baby and if this is so then you need to get advice from a doctor. I cannot guarantee that a dr will help you because you are so young and dr's don't want to help kids under the age of 21. You and your fiancé need to be checked out because some men, especially men have low sperm count and this may be the reason that you cannot convceive. It may be a problem with you as well but you need to get checked out to see what is going on. If you have the money and the responsibility to make and have a baby then you have the responsibility to see a doctor first.
If you want to know when exactly you should conveive buy a ovulation kit at a drug store and it will help you to get prengnant sooner. Also wait before having sex a few days to a week at a time because it helps the guys sperm count to build up. For instance if you have sex on a Monday wait a week for your fiancés sperm to build up. Do not have sex everyday hoping to get prengnant because the sperm doesn't have time to build up and that is low sperm count. Try to wait a week or two before having sex and then have sex on your ovulation time and it will be easier to get pregnant. Ovulation time varies and usually occurs a week or two weeks after your last period and try and have sex on this date.
Here are some other tips on trying to conceive:
Don't pee right after sex
Lay down for a while on you back after sex
Lay down on your back with your feet up in there after sex (helps sperm to swim up to the egg) for 10-15 minutes.
There is also some health herbs that you can buy from your health store that promotes pregnancy or you can buy some online as well.
Also since you are trying to get pregnant you NEED to buy a pre-natal vitamin at the department store like walmart etc. This is good for when or if you get pregnant and also helps to PRMOTE pregnancy. It is actually easier to get prengnant if you are taking this pill ahead of time and is good for the baby. It WILL NOT hurt if you take the pill even though you are not yet prengnant.
I have given you some really GOOD suggestions and I really hope that you will buy those products because if you have the money to have a baby, you have the money to buy the pre-natal vitamins for the baby and in preparation for a healthy baby and yourself. You are 18 and you should not worry yet whether you can have a baby.
You should be enjoying your life as a young adult because you have plenty of time to decide to have a baby. It takes some people 6 months to get pregnant but it usually takes about a year or two of having sex to have a baby and so DO NOT worry, you have plenty of time. Enjoy your free time with your life now while you have a chance and you will have a baby when God is ready for you to. It is Gods decision when to give life to your baby and when he thinks that you are ready you will have one. People do not understand that God promotes life and without God you couldn't have babies and si it is his decision and when he wants you to have kids you will and not before then. Even if it takes 10 years you will have it when it is your time!